Key points are not available for this paper at this time.
We use the Eagle simulation to study the relationship between the stellar haloes of Milky-Way-mass galaxies and their recent merger histories. The stellar mass ratio of the most massive merger that occurred since z=1 is strongly correlated with the z=0 fraction of ex situ stars in the galaxy, f ₄ₗ\ ₒ₈ₓₔ, but is weakly correlated with stellar halo mass fraction, f ₒ₇, particularly for disc galaxies. Contrary to common belief, our results suggest that disc galaxies with low mass stellar haloes do not necessarily have quiescent merger histories; in fact, roughly one quarter have experienced a merger with stellar mass ratio > 0. 1 since z=1. We demonstrate that the population of disc galaxies with low f ₒ₇ and active merger histories undergo mergers with satellites whose orbits are more circular than average and are approximately co-planar with the disc; instead of contributing significantly to the stellar halo, these mergers lead to discs that contain a substantial fraction of ex situ stellar mass and are thicker and more extended than those of quiescent galaxies. Such mergers also supply fuel that often incites a significant episode of in situ star formation in the disc. Our results suggest that seemingly quiescent disc galaxies with low-mass stellar haloes actually have diverse merger histories, which limits the extent to which stellar haloes alone can act as observable tracers of the hierarchical assembly of galaxies.
Proctor et al. (Tue,) studied this question.